Method and apparatus for automatic focusing of scanning electron microscope

ABSTRACT

Method and an apparatus for automatically moving the focal plane of the electron beam produced by a scanning electron microscope to the position of specimen surface. An auxiliary coreless coil of a low inductance is disposed close to the objective lens. The exciting current fed to the coil is varied to move the focal plane of the electron beam toward the optical axis. At each of discrete positions of the focal plane, a specimen is scanned with the electron beam. The resulting secondary electrons are detected by a detector. Individual output signals from the detector are compared to select the maximum signal. Then, only the objective lens is excited. The exciting current fed to the objective lens is modified to move the focal plane along the position closest to the specimen surface.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus forperforming automatic focusing action in a scanning electron microscope.

BACKGROUND OF THE INVENTION

Several methods have been adopted to automatically adjust the focus inscanning electron microscopes. One of them is illustrated in FIG. 5.

As shown in FIG. 5, a deflection unit 14 receives a timing signal from acontrol unit 5. The deflection unit 14 creates a horizontal, orX-direction, scanning signal and a vertical, or Y-direction, scanningsignal according to the timing signal, and supplies these signals to adeflection coil 2. The control unit 5 produces a digital signal forindicating the value of the exciting current fed to an objective lens 3.This digital signal is converted into analog form by a digital-to-analog(D/A) converter 15 and sent to a driver circuit 16. The electron beam 1emitted by an electron gun (not shown) is deflected in the X- andY-directions by the deflection coil 2. The beam 1 is focused by theobjective lens 3, and a desired region on a specimen surface 4 isscanned with this focused beam 1. During this scan, the value of theexciting current supplied to the objective lens 3 is changed inincrements of ΔI within a given range. In particular, as shown in FIG.6, whenever a given time T₀ elapses, the exciting current is varied byΔI. During the given time T₀, the electron beam 1 raster scans thespecimen surface for obtaining one frame of image. The secondaryelectrons released from the specimen surface 4 during this scan aredetected by a detector 6 and converted into an electrical signal, whichis sent to a signal-detecting unit 7 having a low-pass filter 8 and ahigh-pass filter 9. These filters 8 and 9 remove noises from theincoming signal. The signal from which the noises have been removed issupplied to an absolute value circuit 10 which takes the absolute valueof the input signal. An integrator circuit 12 integrates the signalpassed through the absolute value circuit 10 which corresponds to oneframe. The output signal from the integrator circuit 12 is convertedinto digital form by an analog-to-digital (A/D) converter 13 and fed tothe control unit 5. It is assumed that when the exciting current fed tothe objective lens assumes values I₁, I₂, I₃, . . . I_(n), theintegrated values supplied to the control unit 5 are S(I₁), S(I₂),S(I₃), . . . S(I_(n)), respectively. The control unit 5 calculates therelation between the exciting current and the integrated value from theabove values, and then finds the objective lens-exciting current valueI₀ (FIG. 7) giving the maximum integrated value from the calculatedrelation. This current value I₀ is taken as the exciting current whichprovides the focused condition. Therefore, the objective lens is set atthis current value I₀ to achieve focusing.

In the above-described prior art automatic focusing system, focusing isattained by evaluating the output signal from the secondary electrondetector, the output signal being derived immediately after theobjective lens current is varied in an increment as shown in FIG. 6.This makes it impossible to accomplish accurate focusing. Specifically,the exciting coil of the objective lens has a large inductance.Therefore, if the current is varied, the current flowing through theexciting coil of the objective lens does not vary instantaneously;rather it changes gradually as indicated by the dotted lines in FIG. 6.Accordingly, if one evaluates the output signal from the secondaryelectron detector and judges the focused condition immediately after theexciting current is varied as mentioned previously, the focusedcondition is not maintained when the exciting current settles intostationary state.

One contemplated method for solving this problem is to detect andevaluate the output signal from the secondary electron detector afterthe amplitude of the exciting current fed to the objective lens hassettled into stationary state. Then, the exciting current providingfocused condition is determined. However, it is time-consuming toperform automatic focusing action by this method.

Scanning electron microscopes free of this problem have been developed.In particular, a small auxiliary coil is disposed close to the objectivelens. The exciting current supplied to this auxiliary coil is varied tosearch for the focus. One example of such scanning electron microscopesis disclosed in U.S. Pat. No. 4,199,681. In this disclosed microscope,if the exciting current to be supplied to the auxiliary coil forachieving focused condition is determined, then the focusing operationis completed while this current is kept supplied to the auxiliary coil.However, if an electrical current close to the maximum allowable currentvalue has already flowed through the auxiliary coil when the focusingoperation is completed, then it is not possible to widen the searchedarea in the direction to increase the current according to the changingposition of the specimen. Eventually, it is only possible to search anarrow area for the focus. This type of electron microscope has anotherdisadvantage. Specifically, if the electrical current flowing throughthe auxiliary coil is relatively large after the completion of thefocusing action, a slight deviation of the axis of the auxiliary coilfrom that of the objective lens produces a distortion in the image. Thismakes it impossible to observe good-quality images.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method of automaticallyfocusing a scanning electron microscope quickly and accurately.

It is another object of the invention to provide an apparatus whichquickly, accurately, and automatically focuses a scanning electronmicroscope.

It is a further object of the invention to provide a method andapparatus which are used for automatic focusing of a scanning electronmicroscope having an auxiliary coil of a low inductance and whichpermits images to be displayed without being affected by the deviationof the axis of the auxiliary coil from the axis of the objective lens.

It is still another object of the invention to provide a method and anapparatus which permit a scanning electron microscope having anauxiliary coil of low inductance to search a wide area for the focus forachieving automatic focusing.

Other objects and features of the invention will appear in the course ofthe description whereof which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a scanning electron microscope according tothe invention;

FIG. 2 is a flowchart illustrating a method of automatically focusingthe electron microscope shown in FIG. 1;

FIG. 3(a) is a diagram in which the position of the focal plane in themicroscope shown in FIG. 1 is plotted against time for illustrating themanner in which the focal plane is moved by exciting the objective lensalone;

FIG. 3(b) is a diagram similar to FIG. 3(a), but obtained when the focalplane is moved by exciting the auxiliary coil;

FIG. 4 is a graph in which the exciting current fed to the objectivelens of the electron microscope shown in FIG. 1 is plotted against thedistance WD and also with regard to addresses in a memory;

FIG. 5 is a block diagram of the prior art scanning electron microscope;

FIG. 6 is a graph in which the exciting current fed to the objectivelens of the prior art microscope shown in FIG. 5 is plotted against timefor illustrating the disadvantage that the prior art microscope shown inFIG. 5 entails; and

FIG. 7 is a graph in which integrated value is plotted against theexciting current fed to the objective lens for showing the peak of theintegrated value, the integrated value being used as an evaluatingsignal in determining the position of the focus.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a scanning electron microscopeaccording to the invention. FIG. 1 is similar to FIG. 5 except that anauxiliary coreless coil 20 is disposed close to the objective lens 3.The number of turns of wire forming the coil is limited to reduce theinductance. The auxiliary coil 20 is disposed close to the objectivelens 3 such that the magnetic field produced by the coil 20 may besuperimposed on the magnetic field set up by the objective lens. Anexciting current is supplied to the auxiliary coil 20 from an auxiliarycoil driver circuit 22. To specify the value of the exciting current, ananalog signal is sent from a D/A converter 21 to the auxiliary coildriver circuit 22. A memory 23 is connected with the control unit 5.Tables of data for specifying the value of the exciting current fed tothe objective lens 3 are stored in the memory 23.

When the start of automatic focusing action is ordered, the control unit5 makes the distance WD between the principal plane of the objectivelens 3 and the focal plane 11 of the electron beam equal to a standardvalue WD₀. The electron beam is focused most sharply on the focal plane.For this purpose, the control unit 5 supplies standard current I₀ shownin FIG. 4 to the objective lens 3. FIG. 4 also illustrates the contentsof the tables of data described above. In FIG. 4, the horizontal axisindicates the addresses in the memory 23 corresponding to individualvalues of the distance WD. The vertical axis indicates those values ofthe exciting current fed to the objective lens which are necessary tobring the focal plane 11 into the locations corresponding to the aboveindividual values of the distance WD when only the objective lens isexcited. The standard current I₀ is supplied to the objective lens 3.Under this condition, the control unit 5 shifts the focal plane 11 ofthe electron beam along the optical axis by varying the current suppliedto the auxiliary coil 20. FIG. 3(b) shows the movement of the focalplane caused by the auxiliary coil. In FIG. 3(b), the amount of shift ofthe focal plane is plotted on the vertical axis, while time is plottedon the horizontal axis.

In the first search for the focus, the control unit 5 varies theexciting current fed to the auxiliary coil 20 so that the amount ofchange in the distance WD may vary from +Z₁ to -Z₁ (mm). This sweep ismade during a period of T₁ (step 1). The position of the focal plane isshown to vary continuously in FIG. 3(b). In practice, however, theposition of the focal plane changes in a stepwise fashion and isswitched between 32 locations. Whenever the auxiliary current fed to theauxiliary coil 20 assumes a value corresponding to each different one ofthese locations, the electron beam 1 scans the specimen 4 to obtain oneframe of image. The range of movement of the focal plane is setrelatively narrow, because if large electrical currents flow through theauxiliary coil 20, then the response becomes slow. Also, the effects ofthe deviation of the axis of the auxiliary coil from the axis of theobjective lens become more conspicuous. When the first search for thefocus using the coreless auxiliary coil 20 ends and the collection ofdata, or integrated value, is completed, the control unit 5 finds thecondition in which the integrated value assumes its peak value. Underthis condition, the position of the focal plane 11 is closest to thespecimen surface (step 2). This condition is indicated by P₁ in FIG.3(b). The control unit 5 calculates the value ΔI of the exciting currentfed to the auxiliary coil under this condition. Since a given relationexists between the exciting current fed to the auxiliary coil and theamount of shift of the focal plane, i.e., this amount is a function ofthe exciting current fed to the auxiliary coil, the control unit 5converts data indicating the exciting current value ΔI into dataindicating the amount of shift of the focal plane caused by theauxiliary coil 20. Then, the control unit 5 finds an exciting currentvalue of the objective lens which is needed to cause the focal plane ofthe electron beam to move the distance ΔZ_(S1) by exciting the objectivelens alone. It is now assumed that the standard exciting current I₀ isstored at address 1000 in the memory. In order to shift the focal planeby an amount corresponding to the amount of shift ΔZ_(S1), the controlunit 5 reads the value stored at the address which is shifted withrespect to address 1000 by an amount corresponding to the amount ofshift ΔZ_(S1). The objective lens 3 is excited with the exciting currentof the value read out in this way (step 3). Meanwhile, the control unit5 reduces the exciting current fed to the auxiliary coil 20 down tozero. Subsequently, the control unit 5 waits for a time T₂, i.e., untilthe strength of the magnetic field developed by the objective lens 3becomes stable (step 4). As a result, the objective lens is excited withthe exciting current of value I₀ +ΔI_(OL1). That is, only the objectivelens 3 is excited. At this time, the position of the focal plane changesfrom 30 to 31 in FIG. 3(a). Since the objective lens shows hysteresis,the amount of shift of the focal plane often deviates from ΔZ_(S1).Therefore, further search for the focus is necessary. In FIG. 3(a), theposition of the focal plane assumed when only the objective lens 3 isexcited is plotted on the vertical axis, whereas time is plotted on thehorizontal axis.

After the waiting, the control unit 5 makes the second search for thefocus, using the auxiliary coil 20 (step 5). Also in this second search,each time each individual exciting current is supplied to the auxiliarycoil 20, the output signal from the detector is integrated to obtain oneframe of image. This search persists for a time T₃. After the end ofthis search, the control unit 5 selects that value of the auxiliarycoil-exciting current which provides the maximum integrated value (step6). We now assume that the integrated value assumes its maximum value atpoint P₂ in FIG. 3(b) and that the exciting current fed to the auxiliarycoil at this point P₂ is I_(S2). The control unit 5 makes a decision todetermine whether I_(S2) is less than a given reference value K or not(step 7). If the result of this decision is that I_(S2) is less than K,then this automatic focusing process is ended. If I_(S2) is greater thanK, step 3 and the following steps are repeated. FIGS. 3(a) and 3(b )show the latter case, i.e., step 3 and the following steps are repeated.In this case, the optimum amount of shift ΔZ_(S2) of the focal planethat is found by the second search is replaced by a change in theexciting current fed to the objective lens 3. As a result, the excitingcurrent supplied to the objective lens 3 takes a value I₀ +ΔI_(OL1)+ΔI_(OL2) shown in FIG. 4.

Then, the control unit waits until the strength of the magnetic fieldgenerated by the objective lens 3 becomes stable, i.e., for a time T₄(step 4). The third search for the focus is made for a time T₅, usingthe auxiliary coil 20. It is assumed that the optimum amount of shift ofthe focal plane which is found by this search and caused by theauxiliary coil 20 is Z_(S3) and that the exciting current fed to theauxiliary coil 20 under this condition is I_(S3). If the result of thedecision made in step 7 is that I_(S3) ≦K, then the control unit 5maintains the exciting current fed to the objective lens 3 at theabove-described value I₀ +ΔI_(OL1) +ΔI_(OL2). At the same time, theexciting current supplied to the auxiliary coil 20 is set to I_(S3).Then, this automatic focusing process is ended.

It is to be understood that the foregoing constitutes only oneembodiment of the invention and that various modifications and changescan be made in practicing it.

In the above example, if the relation I_(S3) ≦K holds, the control unit5 no longer changes the exciting current fed to the objective lens.Instead, the exciting current fed to the auxiliary coil is set to I_(S3)so that the focal plane may be shifted by Z_(S3) by means of theexcitation of the auxiliary coil. Then, the focusing process is ended.More specifically, in the above example, the exciting current to theauxiliary coil is not at level 0 when the focusing action is completed.The image quality is not affected if the auxiliary coil is kept excitedduring observation of an image, since the value K is selected to besufficiently small. However, it is also possible to reduce the excitingcurrent fed to the auxiliary coil down to zero and to modify theexciting current fed to the objective lens once again before the end ofthe focusing operation.

Also in the above example, a coreless coil is used as the auxiliary coilof a low inductance. If the inductance is sufficiently low, the coil isnot always restricted to the coreless type.

In the above example, the absolute value of the output signal from thesecondary electron detector is taken and then integrated to find theposition closest to the focal point. If the degree of the approximationto the focal point can be judged, other signal processing may also beeffected. In one of such processing, the output signal from the detectormay be differentiated, and then the absolute value of the differentiatedsignal may be taken. The absolute value is subsequently integrated.

In the above example, the amount of shift of the focal plane is foundfrom the value ΔI of the exciting current fed to the auxiliary coil. Ifthe control unit 5 first plots the integrated value against the positionof the focal plane to know the relation between them then it is notnecessary to find the value ΔZ_(S1) or other value from the excitingcurrent value ΔI; rather ΔZ_(S1) or other value can be found directly.

In the above example, a step where the exciting current fed to theauxiliary coil was reduced to zero is inserted before the second searchis started. However, this step can be eliminated.

In accordance with the present invention, the auxiliary coil is used tosearch for the focus. Correspondingly, the exciting current fed to theobjective lens is modified. Hence, large currents do not flow throughthe auxiliary coil after the focused condition has been realized.Therefore, if the axis of the auxiliary coil deviates from the axis ofthe objective lens, astigmatism is not present when observation of thespecimen is carried out. In other words, the image quality does not dropif the axis of the auxiliary coil deviates from the axis of theobjective lens.

Having thus described our invention with the detail and particularityrequired by the Patent Laws, what is claimed to be protected by LettersPatent is set forth in the following claims.

What is claimed is:
 1. A method of automatically focusing a scanningelectron microscope emitting an electron beam and having an objectivelens and an auxiliary coil of a low inductance, the auxiliary coil beingdisposed close to the objective lens, said method comprising the stepsof:(a) varying the exciting current fed to the auxiliary coil to movethe focal plane of the electron beam along the optical axis; (b)scanning a specimen with the electron beam at each different position ofthe focal plane of the beam; (c) evaluating the resulting signals andfinding the position of the focal plane closest to the specimen surface;and (d) modifying the exciting current fed to the objective lens to movethe focal plane toward said position closest to the specimen surface byexciting the objective lens alone.
 2. The method of claim 1, wherein theabove steps (a)-(d) are repeated to effect automatic focusing.
 3. Themethod of claim 2, wherein the value I_(S) of the exciting current fedto the auxiliary coil at said position closest to the specimen surfaceis compared with a reference value, and wherein if this value I_(S) isless than the reference value, then the auxiliary coil is excited withthe exciting current of the value I_(S), whereby the focusing operationis ended without carrying out the step (d).
 4. The method of claim 1,wherein said auxiliary coil of a low inductance is a coreless coil. 5.An apparatus for automatically focusing a scanning electron microscopeemitting an electron beam and having an objective lens, said apparatuscomprising:a scanning means for scanning the surface of a specimen withthe electron beam focused by the objective lens; an auxiliary coildisposed close to the objective lens; a means which varies the currentfed to the coil to move the focal plane of the electron beam along theoptical axis, for searching for the position closest to the specimensurface; a means for evaluating the signal obtained by the search andfinding the amount of shift of the focal plane caused by the auxiliarycoil at the position closest to the specimen surface; and a means forchanging the exciting current fed to the objective lens according to asignal indicating the amount of shift of the focal plane.
 6. Theapparatus of claim 5, wherein said means for evaluating the signalsobtained by the search and finding the amount of shift of the focalplane comprises: a means for finding the value of the exciting currentfed to the auxiliary coil when the focal plane of the electron beam isclosest to the specimen surface; and a means for converting a signalindicating the value of the exciting current into a signal indicatingthe amount of shift of the focal plane.
 7. The apparatus of claim 5,wherein said auxiliary coil of a low inductance is a coreless coil